It is quite common on construction sites to use devices for detecting underground structures before or while digging. Such structures often occur in form of services for supplying electricity, gas, fuel, water, or communication data among other conductive, underground structures. Although the location of most of these services is already known from a surveyor's plan of the site, their locations can be uncertain or there could be additional services that are not mentioned therein. Often they are also simply overlooked by the operator of an earth moving machine during work.
The avoidance of damage to underground structures while digging in a trench or in areas being excavated is an important task. As damage to a service can cause serious impact and costs, additional measurements are taken to be able to detect the proximity, especially as an information of depth, of such services on the site before or while earth moving. Devices for this purpose are known as Cable Avoidance Tools also called CAT.
One way to locate underground services is to detect electromagnetic fields sent out by the service itself. To do this the service requires having a naturally occurring electrical signal which emits an electromagnetic field that is detectable above the ground. This works well for a switched on power supply line or a communication-cable being in use, but for example a wiring system of switched off street lights, unused or low-voltage communication cables, gas- or water-pipes can hardly be detected in this way.
Many different techniques have been developed for detecting those non-signal-carrying types of service as mentioned above as well. If the service itself is electrically conductive, an electromagnetic field can be introduced artificially as described in U.S. Pat. No. 4,438,401, which discloses a system wherein metallic services with no naturally occurring signals are directly connected to an electrical signal-generator. In this way an electrical signal can be coupled to the service, and therefore it is also possible to detect it by electromagnetic fields.
If this is not possible since the service is non-conductive, U.S. Pat. No. 5,194,812 shows a solution for making hollow pipes like gas or none-metal water pipes detectable by introducing a conductor into them—or by laying a conductor next to the service when it is buried—that will function as a transmitter for electromagnetic fields. In this way also a pipe not having a naturally occurring field by itself can be detected similar to conductive services by applying a detectable signal to the conductor inside or next to the structure.
As shown in the application EP 09166139 it is also possible to couple a field emitting signal into a conductive underground structure by introducing a current from an AC current-source into soil by earth-spikes or other ground connection means. As the current always takes the path of least resistance through soil, it will mostly flow along the conductive structure, since its resistance is lower than those of the soil-material. This way a conductive structure is detectable by its electromagnetic field although it neither naturally emits such, nor can a signal be directly conducted to it.
A further possibility for applying a signal to an underground service is to emit an electromagnetic field into the ground by a transmitting aerial. A receiving aerial or sensor in the detection device can then pick up the radio-signals reflected back by the underground structure, and, based on this information, the proximity of a service can be determined. The documents U.S. Pat. Nos. 4,600,356, 5,592,092 and 6,437,726 show such devices that for example are mounted on the bucket of a backhoe.
What all the mentioned detection systems have in common is that the underground structures need to emit an electromagnetic field that is strong enough to be detectable above the surface; especially it has to be detectable non-ambiguously in respect of the always present noise-floor of various other electromagnetic fields from other sources.
The electromagnetic fields emitted by the different services reside in different ranges of frequency dependent on the signals present on the service.
Power-lines commonly provide currents with a fundamental frequency of 50 Hz or 60 Hz, dependent on the country, and therefore emit fields with the same fundamental frequency.
In contrast thereto, the signals that are artificially applied to the structures (either by direct or by soil connection) are restricted in frequency by radio-communication-rules which are country-dependent and given e.g. to avoid interferences with radio communication services. In this case the field emitted by the structure has a frequency specified by the known signal being artificially introduced to the structure. A special example of frequencies allowed in the UK for a general geographic surveillance use, such as cable detection, are the frequencies of 8 kHz or 33 kHz, which are used by some CAT-equipment.
The frequency spectrum that is emitted from communication lines is another important detection target. The fields from such services can be expected to occur in certain ranges, whereby for those services no special single frequency can be expected, but rather a range of frequencies has to be taken into account. Especially the low wavelengths in the range of myriameter are recommended for geophysics since they penetrate soil material quite well and can therefore also be used for cable detection purpose.
To cover all those frequency ranges mentioned above, the known devices provide different modes of operation that can be exclusively selected of:                Power mode of operation, targeting electromagnetic fields from the alternating current flowing through power lines with a fundamental frequency of 50 Hz or 60 Hz.        Radio mode of operation, targeting electromagnetic fields in the VLF radio band range (=Very Low Frequency radio waves e.g. in the range of about 15 kHz to 60 kHz) as emitted by communication lines or similar services.        Active mode of operation, targeting an electromagnetic field of a known frequency which is specified by a signal being actively applied to the structure by radio, soil- or direct-conduction or by the introduction of a transmitting conductor inside of the structure, e.g. a 8 kHz or 33 kHz current applied to the structure by a current source.        
The document U.S. Pat. No. 4,085,360 mentions a receiver instrument which consecutively or simultaneously listens to different frequency bands, one for mains at 50/60 Hz and one wide band from 1 kHz to 10 MHz.
US 2004/0189322 discloses a cable detection apparatus having a comb filter which is transmitting odd and attenuating even harmonics of the first frequency of the signal to be detected. By analysing fundamental and harmonics of the desired frequency, the SNR is improved.
A reason for the usage of different modes of operation is the fact that the accuracy of the proximity detection is strongly dependent on the signal to noise ratio of the signals being evaluated. As described in GB 2 427 473, the noise is scaling with the square of bandwidth and good detection results can be achieved when using a narrow band pass filter that dampens all frequencies except the ones of interest for the measurement (e.g. the 50 Hz or 60 Hz of mains in Power Mode or the 8 kHz or 33 kHz in Active Mode). The filtering can be done analog or digital, often also a combination of both kinds of filtering is used. A narrow band pass filter in this case is defined as a filter of a bandwidth which is lower than 10% of the centre frequency value, in particular lower than about 30 Hz.
A drawback of the narrow filtering on the other hand is that the area of interest has to be scanned more than once, each time for one of the different modes of operation for detecting all types of services in the small frequency bandwidth. This can be a time consuming and expensive process, especially when done by an experienced craftsman.
To overcome this drawback, there are devices known such as disclosed in U.S. Pat. No. 7,403,012, which contain three different detection paths in parallel in a single device. This approach is rather complex, power consuming and also expensive to build, since the whole signal conditioning and evaluation path—except the antennas that might be shared—has to be present three times.